Distributed generation, also commonly referred to as on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy, generates electricity from multiple small energy sources. The distributed power generation system comprising the multiple small energy sources is interconnected with the same transmission grid as the larger central power generation stations. Various technical and economic issues arise in the integration of these distributed generation resources into the grid, such as power quality, voltage stability, presence of harmonics and reliability.
Grid-interactive converters/inverters with energy storage system (ESS) for distributed power generation (DG) systems have been gaining popularity in response to the requirements for energy source/storage diversity, environmental concerns, desired cost reduction, improvements in efficiency, etc. One main challenge in DG systems is determining how to improve the efficiency, reduce the cost and improve the power quality of the current DG systems known in the art.
Traditionally, AC link systems and DC link systems have been employed in distributed generation systems known in the art. In the DC link system, as shown in FIG. 1(a), multiple DC-DC converters are used to interface different energy sources/storages. In order to achieve the DC-AC energy conversion, the DC-DC converters are usually connected to one inverter with a common DC bus. As a result, the DC link system configuration still requires DC-DC and DC-AC conversion stages. Limited switching frequency on the common inverter results in the requirement for a large AC filter and large electrolyte capacitors, which seriously impacts the overall system efficiency, cost and lifetime of the inverter. Furthermore, in order to achieve desired wide range reactive power compensation, the DC capacitor and inverter must be oversized to generate the possible AC output voltage.
As shown with reference to FIG. 1, in the traditional AC link system, multiple DC-DC converters and DC-AC inverters are used to interface different energy sources/storages as shown in FIG. 1(b). Though the real and reactive power can be distributed flexibly between different energy sources/storages utilizing the AC link system, the system requires multiple energy conversion stages. Moreover, due to the common AC bus used in the AC link system, the power flow control must to be carefully designed, particularly to address the needs of weak power systems. In an AC link system, the overall system cost will increase due to the need for multiple converters and inverters and large passive components. In addition, the transformer utilized in both the DC link and the AC link systems described above will increase the overall system size and reduce the power density.
Accordingly, what is needed in the art is a distributed generation (DG) system that exhibits high power density, high power efficiency, high power quality and high system reliability.